The present invention relates a method for making random copolymers and toner materials which incorporate such copolymers. The invention also relates to copolymers and toners made by this method.
In the prior art, randomizing agents and polar modifiers such as N,N,N',N'-tetramethylenediamine (TMEDA), tetrahydrofuran (THF) and 1,2-dipiperidinoethane (DPE) have been used to produce poly-1,2-butadiene with more than 95% 1,2-vinyl groups while using organolithium initiators such as n- or s- butyllithium. Halasa et al., Advances in Organometallic Chemistry, Vol. 18, No. 55 (1980), have shown that in reactions involving butadiene, less than 35 wt % styrene and 2 equivalent moles of TMEDA or DPE per mole of organolithium initiator group, the reaction rate is accelerated compared with polymerization rates without polar modifiers. Moreover, 1,2-vinyl content of the product is increased markedly and monomer insertion in the polymer chain approaches a nearly random condition. However, the polymers produced by Halasa et al. are not suitable for use in toners. The styrene content of these polymers is so low that they are not brittle enough to be formed into toner particles in a commercially feasible operation.
U.S. Pat. No. 4,469,770 discloses a method of making a toner comprising a styrene butadiene copolymer. The copolymer contains from about 85 to about 93 weight percent styrene and from about 7 to about 15 weight percent butadiene. Copolymers which are used in toners are often produced by free radical polymerization, as described in U.S. Pat. No. 3,634,251.
Block copolymers which have been reported to be suitable for use in toner compositions were disclosed in U.S. Pat. Nos. 3,965,021, 3,967,962 and 4,528,257. Graft copolymers have also been used in toner compositions as described in U.S. Pat. No. 4,385,107. A toner composition comprising a copolymer is described in U.S. patent application Ser. No. 621,704 filed Dec. 3, 1990, now U.S. Pat. No. 5,166,026, and U.S. patent application Ser. No. 587,194 filed Sep. 24, 1990, now U.S. Pat. No. 5,158,851.
In the absence of polar modifiers and at low styrene-to-butadiene monomer ratios, butadiene polymerizes preferentially compared with styrene, high 1,4-butadiene enchantment results, and graded block copolymers are formed with enriched styrene tails. Block copolymers of polystyrene and poly 1,4-butadiene have many advantages for specific elastomer applications. However, random, nearly random and very short block copolymers are preferred as the chief resin component in toners. Moreover, incorporation of 1,2-butadiene segments in the polymer chain allows an increased amount of butadiene to be used in the styrene copolymer structure without loss in the brittleness required for processing into small toner particles. Copolymers with decreased styrene content usually fuse at lower temperatures.